The present invention relates to a scanning electron microscope, and more particularly to a scanning electron microscope which is suitable for observing semiconductor wafers with a high resolution in the course of inspecting the wafers during their production.
Disclosed in Japanese Patent Laid-Open No. 63-254649(1988) is a scanning electron microscope in which a specimen is irradiated with a light beam emitted from a light source, such as a laser, and the reflected light beam from the specimen is detected by a position detector such as a linear sensor. The output of the sensor is used to produce an electric signal representative of the moving distance of the specimen, in the direction of an electron beam, or the height of the specimen, and the produced signal is used to control a focusing electron lens so as to focus the electron beam on the specimen.
However, for a specimen, such as a semiconductor wafer, which has optical disturbances on its surface, including a difference in reflectance, the measured result includes an error due to the optical disturbances. This is because the intensity, or the intensity distribution, of the light beam detected by the sensor is changed where there is a difference in reflectance depending on the position, due to a circuit pattern formed on the wafer.
In this connection, in the arrangement disclosed in Japanese Patent Application Laid-Open No.57-139607 (1982), the measuring accuracy of the height of the specimen is improved by reducing the light spot diameter on the specimen and by feedback-controlling the light source so that the amount of light incident on the detector always becomes equal to a pre-set reference value in order to reduce any change in the light intensity distribution.
When the light beam is incident on the specimen such as a semiconductor wafer having optical disturbances, especially, fine pits and projections, such as formed by circuit patterns on its surface, the optical disturbances scatter the reflected light to disturb the result detected by the sensor. As a result, the focusing cannot be carried out with a high accuracy.
Further, even if the light spot diameter on the specimen is reduced and the light source is feedback-controlled so that the amount of light incident on the detector always becomes equal to a pre-set reference value in order to reduce any change in light intensity distribution, it is impossible to remove the error contained in the measured result caused by the scattering of the reflected light due to the pits and projections on the surface of the specimen.